Alloy steel and method



United States Patent 3,183,081 ALLOY STEEL AND METHOD William C. Clarke, Jr., Baltimore, and George N. Goller, Towson, Md., assignors to Armco Steel Corporation, Middletown, Ohio, a corporation of Ohio No Drawing. Continuation of application Ser. No. 468,528, Nov. 12, 1954. This application June 1 1964, Ser. No. 375,927

9 Claims. (Cl. 75-128) Our application for patent is a continuation of our copending application Ser. No. 468,528, filed November 12, 1954, and entitled Alloy Steel and Method, and the invention relates to stainless steel, and more especially to a stainless steel of high yield strength and to a method of heat-treating the same.

One of the objects of our invention is the provision of a comparatively inexpensive austenitic stainless steel which readily lends itself to various working, forming and fabricating operations, and which then may be hardened to develop high yield strengths and low magnetic permeability.

Another object is the provision of such a steel which may be cut, bent, stamped, machined or otherwise formed and fabricated into a host of articles which are essentially non-magnetic and which may be heat-treated by precipitation methods to give desired strength at room temperature.

A further object is the provision of various articles such as non-magnetic mine sweeper parts, retainer rings for electric generators and the like which are hard, strong, of high yield strength, tough, non-magnetic and yet comparatively inexpensive, and other articles such as petroleum well-head fittings, valve parts, springs and the like, which are resistant to corrosion, cracking and embrittlement in the presence of hydrogen sulphide.

Other objects of our invention in part will be obvious and in part pointed out thereinafter. The invention accordingly consists of a combination of elements, composition of ingredients and articles fashioned thereof, as well as in the method of heat-treating the same as described herein, the scope of the useful application of all of which is indicated in the claims at the end of this specification.

As conducive to a better understanding of certain features of our invention it may be noted that the nonmagnetic stainless steel commonly comprises about 10% to 35% chromium, 7% to 30% nickel, and remainder substantially all iron. Other ingredients, of course, may be present in minor amounts in order to secure special benefits.

The chromium-nickel stainless steels, of course, are critically strategic because of the strategic character of the nickel addition. Moreover, in many applications the yield strength is inadequate. These shortcomings are felt in the production of various products of manufacture requiring high strength non-magnetic wrought metal parts, as for example, in mine sweepers, retaining rings for generator rotors, and the like.

An outstanding object of our invention, therefore, is the provision of a stainless steel which employs a minimum of strategic alloying ingredients, which is ductile, malleable and readily lends itself to common forming and fabricating operations and yet which by proper subsequent heat-treating operation will acquire great yield strength with retained non-magnetic characteristics.

And, referring now more partciularly to the practice of our invention, we provide a stainless steel essentially consisting of the ingredients chromium, manganese, nickel, phosphorus, nitrogen and carbon, with remainder iron, all in such relative amounts that the steel is substantially wholly austenitic. And we treat this steel by Patented May 11, 1965 subjecting the same to a solution-treatment to place the phosphorus compounds in solid solution followed by a subsequent precipitation heat-treatment at lower temperature. Where desired, further hardness and strength may be had through an additional precipitation heat-treatment at lower temperature, this evidently developing a secondary precipitation.

In the steel of our invention, and in the hardened and strengthened articles thereof the carbon and phosphorus contents are especially critical as more fully indicated below. We find that with less than the required critical amounts of either the desired hardness is not achieved. And with amounts higher than our maximum permissible figures the steel loses ductility and malleability apparently as a result of the formation of various eutectic phosphides or the formation of eutectics with these phosphides, all with the result that various hot-working properties drastically suffer.

The stainless steel of our invention essentially consists of 12% to 19% chromium, 4% to 20% manganese, 4% to 10% nickel, .15% to 35% phosphorus, .l0% to about .2% carbon, and remainder substantially all iron. Nitrogen is employed in the amount of .10% to .40% to achieve high yield strength, more particularly in the amount of .l0% to about .2%, as appears below. Sulphur may be included in amounts up to .15%. To improve the hot-working properties we preferably includ boron in amounts also up to .005

We consider that our steel is in every sense critical in its composition. The chromium content is fairly critical because with amounts less than 12% the steel, upon coldworking, may become magnetic. And with amounts exceeding 19% we find no benefit; on the contrary, the steel is inclined to become ferritic, and partially magnetic, and to maintain the austenitic balance greater amounts of the austenite-forming elements manganese, nickel and nitrogen are required. Similarly, our ranges of manganese and nickel both are quite critical in that with lower amounts of manganese and nickel there is a loss of austenitic stability, and with greater amounts there is a waste in the case of nickel and excessive attack of furnace refractories in the case of manganese. We find that a minimum of 4% nickel is required in order to achieve a stable austenitic structure.

The critcial nature of the phosphorus content of our steel is indicated above, and so also is indicated the critical nature of the carbon content. The nitrogen, too, is critical in its content because we find that with greater quantities of nitrogen there is a substantial risk of gassy metal, and with lesser amounts the required mechanical properties are not had, as more fully indicated hereinafter. The upper limit of sulphur also is critical in that greater amounts are not readily taken up by the steel apparently because of its high manganese content. And boron, where used, is most critical because where the value substantially exceeds .005% there is a possibility of undesired brittleness with consequent splitting of the metal in hot-working.

Now our steel of the composition indicated is fully austenitic, non-magnetic and readily lends itself to a host of working, forming and fabricating operations. The steel may be forged, swaged, rolled, drawn, pierced and extruded. It may be upset, pressed, stamped and blanked. It may be machined or otherwise cut, turned, ground and the like. In short, our steel readily lends itself to fashioning into a variety of articles of ultimate use.

Moreover, our steel lends itself to'a precipitationhardening treatment which develops high yield strengths. And yet the non-magnetic characteristic of the steel is fully retained.

In treating our steel to develop the desired high yield strengths we first eifect a solution-treatment by heating 4 The surprising mechanical properties of our steel (Heats 7420, 7421, 7422 and 7603), particularly in matters of high yield strength and ductility, as measured by percent elongation and percent reduction in area are given in Table II below, this for all three types of hardening treatment as described above.

TABLE II Mechanical properties at room temperature of the steel of Table I Treat Rock. U.T.S., .2% Y.S., .02% Y S Percent Percent Izod imp. Heat N o ment Hard. p.s.i. p.s.i. p.s.l el. in 2 red. area stfigerligth,

Nora-The treatments A, 13 and C of Table II are as follows:

A-2050 F. 1 hr.-W.Q.+1300 F. 24 hrs.-W.Q. B-2050 F. 1 hr.W.Q.+1300 F. 12 hrs.W.Q. +1200 F. 24 hrs.W.Q. C2050 F. 1 hr.-W.Q.+1300 F. 24 hrs.W.Q.

+1150 F. 2 l1rs.-W.Q.

perature of 1950 F. to 2050 F. because we find that best results are had within this narrow range. And the duration of the treatment at this temperature is on the order of say 15 minutes to several hours, depending upon the size of the metal article or product being treated. Satisfactory results generally are had with a treatment of about one hour. We preferably quench the steel in water from the solution-treating temperature.

We find that our steel develops an exceptionally high yield strength by treating the same, following the solution-treatment, at some 1000 F. to 1400 F. for a period of time ranging from several hours on up to several days. For most products this heat-treatment is conducted at a temperature of about 1300 F. for something on the order of about twenty-four hours. Following this, the products are cooled and are ready for use.

The steel and products of our invention when heattreated as described above have room temperature yield strengths (.2% offset yield) on the order of 120,000 p.s.i. and more. And along with this there is had good ductility and impact strength.

We also find that the yield strength figure may be further increased by subjecting the steel and product to a still further heat-treatment, this at a somewhat reduced temperature. Good results are achieved where the steel following heat-treatment at 1000 F. to 1400 F. and cooling is reheated at 1000 F. to 1200 F. for a shorter period of time, that is, an hour or two on up to about twenty-four hours, and cooled. Apparently this second treatment develops a secondary precipitate and this is had quickly because the structure of the metal is in a strained condition.

As specifically illustrative of the high yield strength steel of our invention, six steels are presented below in Table 1, four of these (Heats 7420, 7421, 7422 and 7603) to be contrasted with two others (Heats 7419 and 7602):

TABLE I Chromium-manganese-nickel-phosphorus stainless steel A comparison of the yield strengths of Heats 7420, 7421 and 7422 of high nitrogen content (.10% to about .2%) with the Heat 7419 of low nitrogen content (an insignificant .031%) reveals the substantial benefits had with the nitrogen addition. And comparison of the samples of Heat 7602 (037% nitrogen) in heat-treated conditions A, B and C, as well as comparison of the three' samples of Heat 7603 (.20% nitrogen) in those three different heat-treated conditions, reveals significant increase in ultimate tensile strength had with the double precipitation-hardening heat-treatment. Moreover, a surprising increase in the yield strength is had, with both single and double treatments, in the steel of substantial nitrogen content, Heat 7603, as compared with Heat 7602.

Our chromium-manganese-nickel-phosphorus steel with or without high nitrogen content possesses exceptional strength at room temperature and high yield strength together with good ductility. Thus it will be seen that we provide in our invention a stainless steel and various articles and products fashioned thereof in which the various objects hereinbefore noted together with many practical advantages are achieved. It will be seen that the steel and products are well adapted to conditions requiring strength in combination with freedom from magnetic effects as more particularly noted above.

As many possible embodiments may be made of our invention and as many changes may be made in the embodiments herein set forth, it will be understood that all matter set forth herein is to be interpreted as illustrative and not as a limitation.

We claim as our invention:

1. Austenitic stainless steel, precipitation-hardenable by heat-treatment to achieve high yield strength, said steel comprising 12% to 19% chromium, 4% to 20% manganese, 4% to 10% nickel, .15%'to .35% phosphorus, .10% to about .2% carbon, nitrogen .10% to .40%, sulphur up to .15 boron up to .005 and remainder substantially all iron.

2. Stainless steel, precipitation-hardenable by heat-treatment to achieve high yield strength, said steel comprising 12% to 19% chromium, 4% to 20% managanese, 4% to 10% nickel, .15% to 35% phosphorus, .10% to about .2% carbon, .10% to about .2% nitrogen, and remainder substantially all iron.

3. Austenitic stainless steel, precipitation-hardenable by heat-treatment to achieve high yield strength, said steel comprising about 17% chromium, about 9% manganese, about 5% nickel, .15% to 35% phosphorus, .10%

to about .2% carbon, .10% to .40% nitrogen, and remainder substantially all iron.

4. In the production of stainless steel of high yield strength the art which comprises preparing a stainless and the remainder substantially all iron; solution-treating the steel at a temperature of 1850 to 2200 F. and quenching; reheating the steel to a temperature of 1000 F. to 1400'F. and cooling; and further reheating the steel to a temperature of 1000 F. to 1200" F.

5. Austenitic stainless steel of high yield strength comprising 12% to 19% chromium, 4% to 20% manganese,

4% to 10% nickel, .15% to .35% phosphorus, .l% to about .2% carbon, .10% to .40% nitrogen, and remainder substantially all iron,-solution-treated and precipitation? hardened. Y

6. Austenitic non-magnetic stainless steel machined 0 articles of high yield strength comprising about 17% chromium, about 9% manganese, about 5% nickel, .15 to 35% phosphorus, .l0% to about .2% carbon, .10%

to .40% nitrogen, and remainder substantially all iron, solution-treated and precipitation-hardened.

,7. Generator retainer rings comprising 12% to 19% chromium, 4% to manganese, 4% to 10% nickel, .15% to phosphorus, .l0% to about .'2% carbon, .10% to .40% nitrogen, and remainder substantially all iron.

8. Petroleum well-head fittings, valve parts, springs, and the like comprising 12% to 19% chromium, 4% to 20% manganese, 4%. to10% nickel, .15%.to 35% phosphorus, .l0% to about .2% carbon, .10% to .40% nitrogen, and remainder substantially all'iron.

9. Petroleum well-head fittings,-valve parts, springs, and the-like comprising. about 17% chromium, about 9% 'manganese, about 5 %"nickel, .15% to .35% phosphorus,

.10% to about .2% carbon, .10% to about .2% nitrogen,

' andremainder-substantially all iron.

References Cited the Examiner. I UNITED STATES PATENTS 2,686,116 I 8/54 861161111 1 et al. -l28 DAVID L. RECK, Primary Examiner. 

1. AUSTENITIC STAINLESS STELEL, PRECIPITATION-HARDENABLE BY HEAT-TREATMENT TO ACHIEVE HIGH YIELD STRENGTH, SAID STEEL COMPRISING 12% TO 19% CHROMIUM, 4% TO 20% MANGANESE, 4% TO 10% NICKEL, .15% TO .35% PHOSPHORUS, .10% TO ABOUT .2% CARBON, NITROGEN .10% TO .40% SULPHUR UP TO .15%, BORON UP TO .005% AND REMAINDER SUBSTANTIALLY ALL IRON. 